1992 Cape Mendocino Earthquake and Tsunami

A geological journey through the eyes of a keen observer, a fictional character, Tatum Dunklin.

Seismograph at Cal Poly Humboldt showing seismic waves arrive. Credit: Mike Dronkers, Cal Poly Humboldt.

An earthquake happened. A tsunami happened. A high school student, Tatum Dunklin, joined a university class to investigate. We follow them on their adventure.

The Beginning of the Journey

As part of Tatum’s Arcata High School science class, they were attending a geology class at  Cal Poly Humboldt  called “Earthquake Country.”

Tatum loved this class because it was created to help non-scientists learn about earthquakes, tsunami, and other natural hazards.

Tatum craved this type of knowledge.

The Earthquake

There was a festival at Redwood Park, uphill from the city of Arcata, on Saturday 25 April 1992.

Tatum had been looking forward to spend some time dancing to music surrounded by a redwood forest.

Arcata Community Forest adjacent to Redwood Park

On the way to the festival, Tatum thought that the car had a flat tire, and stopped to check the tires, but all appeared OK.

Once Tatum arrived at the festival, a group of friends mentioned that there was an earthquake, and all the tall redwood trees were swaying back and forth like grass blowing in the wind.

Upon reflection, Tatum realized that the earthquake shaking is what made the car feel like it had a flat tire.

The Geology Department

Shortly after arriving at the festival, Tatum ran into a friend, Justice, who was a college student in the Earthquake Country class.

After discussing the earthquake, Tatum and Justice went up to the  Geology Department  to see the seismic waves from the earthquake plotted on the seismograph there. Select the image below to enlarge to see the details.

Seismogram as plotted on the Geology Department “Baby Benioff” seismograph. Seismic waves from the magnitude M 7.2, M 6.5, and 6.7 earthquakes are labeled.

While at the Geology Department, Tatum and Justice encountered  a geology professor, Dr. Lori Dengler . Dr. Dengler would later study the tsunami produced by this earthquake.

Dr. Dengler would also later form the  Redwood Coast Tsunami Work Group , an organization devoted to help define the needs of local jurisdictions to mitigate the North Coast earthquake and tsunami hazard and to promote a coordinated, consistent mitigation program for all coastal areas.

Here is a plot of the tsunami as it was recorded in Humboldt Bay, the closest tide gage to the earthquake.

Tide gage plot (a.k.a., marigram) from the North Spit tide gage.

Justice and Tatum then learned that there was a  Cal Poly Humboldt Geology Department  school bus headed out to the field to go search for geological evidence from the earthquake.

After asking permission, Tatum grabbed a water bottle and snacks to join Justice on a journey of discovery. Let’s follow Tatum and Justice on their earthquake adventure.

The Field Trip

 A geology professor, Dr. Gary Carver , welcomed Tatum and Justice while they joined the other students on the bus.

As the bus left from the university parking lot, Dr. Carver began describing the tectonic setting of northern California.

Carver described how there are three main tectonic plates and three main earthquake faults that separate them. The region where these three faults meet is called the Mendocino triple junction, named after Cape Mendocino, a part of the coast of California that sticks out into the Pacific Ocean.

The San Andreas and Mendocino faults are strike-slip faults that slip side by side.

The Cascadia subduction zone is a convergent (pushing together) plate boundary formed as the oceanic crust of the Gorda plate (a small part of the Juan de Fuca plate) "subducts" eastward beneath the continental North America plate.

Between earthquakes, the subduction zone fault is locked and causes the ground to bulge up or down in different places. When the fault slips during an earthquake, the ground generally goes up or down in the opposite direction.

Top: Map showing the tectonic plates and plate boundaries including the Cascadia subduction zone, the San Andreas fault, and the Mendocino fault. The epicenter for the 1992 Cape Mendocino earthquake is shown as a yellow star. Bottom: A cross section A to A’ (with a location shown on the map) showing a sideways view of the plates. On the left is for times between earthquakes and on the right is for times during an earthquake. The plots show areas that go up or go down during these different time periods.

Here is a view of how earthquakes can generate tsunami:

This figure shows how the crust moves between and during earthquakes. We also see how a tsunami can be generated by an earthquake.

Learn more about Cascadia subduction zone earthquakes in the USGS document, “ The Orphan Tsunami .”

The following map shows the major faults in the region and the locations of towns that are part of this journey.

Select the map to learn how the M 7.2 earthquake epicenter (yellow star) is in the region we call the Mendocino triple junction.

Map shows the coastline, roads, and earthquake faults near the Cape Mendocino Earthquake.

We can see that there are many earthquake faults in this area, which is why it is interesting for earthquake geologists.

Click on the faults (black lines) in the map to the right to learn more about the faults.

Tectonics and the Landscape

The shape of the ground surface, the hills and valleys, are formed by these tectonic forces.

As the bus drove southwards on Highway 101 and later crossed the Eel River on the Fernbridge, Dr. Carver pointed out the features that show evidence for this tectonic forcing.

Looking to the East, Dr. Carver showed the students an ancient river terrace, once formed flat along the Eel River, that has been uplifted by tectonic faulting and now tilted to the side, no longer flat and horizontal.

The communities of Fortuna and Rohnerville are located on this uplifted ancient river terrace.

Side view of modern and ancient river floodplains. The modern Eel River floodplain is horizontal and the ancient floodplain, called a terrace, has been tilted upwards because of earthquake faulting beneath the Earth's surface.

In the distance, the students noticed smoke rising across the horizon and would later learn of the fires caused by the earthquake.

There was a large fire in the town of Scotia (along highway 101 south of Rio Dell) where the market burned to the ground.

View of a shopping mall in Scotia, California (population 1,000), destroyed by a fire likely resulted from an electrical short caused by the earthquake.

 Fires  caused by broken gas lines and other factors are one of the most common reasons for building damage following earthquakes. The strong ground shaking caused lots of damage.

Tectonics can alter both natural and human formed landscapes.

The Victorian Village of Ferndale

As the bus rolled safely through the Victorian village of Ferndale, the students looked out the windows and saw houses had shifted off their foundations and some chimneys had fallen away from the houses.

The upper walls of some buildings had collapsed onto cars in the street, crushing them.

Thankfully people did not rush out of those buildings in response to the earthquake!

The brick façade from the Valley Grocery collapsed and smashed two cars. During the 1906 San Francisco earthquake, the façade of the same grocery fell, killing two cows! 

This yellow Victorian house in Ferndale shifted, causing the brick chimney to break into two parts. The gray stripe along the side of the house shows the original position of the chimney.

This red-colored Victorian wood-frame home in Ferndale was damaged severely. The horizontal shift of the house is seen by the relative position of the house and the steps (still standing). The wooden skirting (on its side) indicates the original height of the house above ground level.

Travelling over the Hills

The geology students travelled up and over the Wildcat Hills, across Bear River Ridge, through the Bear River Valley and Capetown, and down to Singley Flat just north of Petrolia (where the first oil well in California was located).

In the map, see how many earthquake faults the buss travels over on its journey. Notice how the fault names are often shared with geographical features on the map.

California's first drilled oil wells that produced crude to be refined and sold commercially were located on the North Fork of the Mattole River approximately three miles east of here. The old Union Mattole Oil Company made its first shipment of oil from here, to a San Francisco refinery, in June 1865.

Sea Cliffs Formed by Tectonics

Here at Singley Flat, just north of Petrolia, there is possible evidence for prehistoric earthquakes.

The modern beach has a sea cliff at its back edge, formed by wave erosion.

If we swipe the control bar left and right to compare satellite imagery with high resolution topographic data, we can see that there is an older sea cliff between the beach and the road.

Geologists hypothesize that this cliff was uplifted by a prehistoric earthquake so that it is now higher in elevation compared to the modern sea cliff.

Sea Critters and their Tectonic Fate

The bus stopped near Singley Flat and the enthusiastic students rifled out of the bus following Dr. Carver.

Upon arrival at the beach, they encountered a local resident who told them that the ocean water level had gone down immediately after the earthquake.

The local resident later observed small tsunami waves.

Dr. Carver leads the students to investigate the uplifted rocks along the coastline. They are walking on rocks that used to be underwater but are now high and dry.

The sea-level was visibly lower than before the earthquake, so it was not just the tsunami that caused the ocean to recede.

The earthquake had caused a large area of the coast to be uplifted! This made it appear that the sea level was lower.

In the weeks following the earthquake, the sea life attached to these uplifted rocks perished.

Dr. Carver and other geologists used the organisms to estimate the change in the land level and identify how much the rocks were uplifted along the coast.

Seismic Uplift and Subsidence

This map to the right shows where uplift (red areas) and subsidence (blue areas) happened during the earthquake.

The 1992 earthquake behaved much like the Cascadia subduction zone behaves as we learned earlier in this journey.

These data are based on high precision surveys of roads and highways in the area in addition to information from the dead sea life.

These geologists determined that along the coast, the ground went up as much as 1 meter (about 3 feet).

Below is a map from the U.S. Geological Survey publication, the source for these vertical land motion data.

This map shows where places went up or down during the earthquake. This vertical land motion is what generated the tsunami recorded on tide gages across the state.

The geology students learned many valuable lessons about our dynamic Earth during the field trip that day.

  1. Earthquake shaking can cause damage and fires in buildings that can potentially injure people.
  2. Earthquakes near the coast can cause tsunamis.
  3. Earthquakes can cause permanent subsidence or uplift of the land.

Acknowledgements

We would like to thank our partners at  Cal Poly Humboldt, Department of Geology , the residents of Humboldt County, and the  Redwood Coast Tsunami Work Group , who all helped contribute to this story. The story is a composite of experiences and observations made by many people.

Drs.  Gary Carver  and  Lori Dengler  are deserved of particular gratitude as they devoted their lives to helping people learn more about earthquake and tsunami hazards posed to the Northcoast of California and beyond.

Arcata Community Forest adjacent to Redwood Park

Seismogram as plotted on the Geology Department “Baby Benioff” seismograph. Seismic waves from the magnitude M 7.2, M 6.5, and 6.7 earthquakes are labeled.

Tide gage plot (a.k.a., marigram) from the North Spit tide gage.

Top: Map showing the tectonic plates and plate boundaries including the Cascadia subduction zone, the San Andreas fault, and the Mendocino fault. The epicenter for the 1992 Cape Mendocino earthquake is shown as a yellow star. Bottom: A cross section A to A’ (with a location shown on the map) showing a sideways view of the plates. On the left is for times between earthquakes and on the right is for times during an earthquake. The plots show areas that go up or go down during these different time periods.

This figure shows how the crust moves between and during earthquakes. We also see how a tsunami can be generated by an earthquake.

Map shows the coastline, roads, and earthquake faults near the Cape Mendocino Earthquake.

Side view of modern and ancient river floodplains. The modern Eel River floodplain is horizontal and the ancient floodplain, called a terrace, has been tilted upwards because of earthquake faulting beneath the Earth's surface.

View of a shopping mall in Scotia, California (population 1,000), destroyed by a fire likely resulted from an electrical short caused by the earthquake.

The brick façade from the Valley Grocery collapsed and smashed two cars. During the 1906 San Francisco earthquake, the façade of the same grocery fell, killing two cows! 

This yellow Victorian house in Ferndale shifted, causing the brick chimney to break into two parts. The gray stripe along the side of the house shows the original position of the chimney.

This red-colored Victorian wood-frame home in Ferndale was damaged severely. The horizontal shift of the house is seen by the relative position of the house and the steps (still standing). The wooden skirting (on its side) indicates the original height of the house above ground level.

California's first drilled oil wells that produced crude to be refined and sold commercially were located on the North Fork of the Mattole River approximately three miles east of here. The old Union Mattole Oil Company made its first shipment of oil from here, to a San Francisco refinery, in June 1865.

Dr. Carver leads the students to investigate the uplifted rocks along the coastline. They are walking on rocks that used to be underwater but are now high and dry.

This map shows where places went up or down during the earthquake. This vertical land motion is what generated the tsunami recorded on tide gages across the state.